Floating Foundation

 What is floating foundation ! 

The Floating Foundation is a type of foundation which is constructed by excavating the soil and earth strata in such a way that the weight of structure built on the soil is nearly equal to the total weight of the soil excavated from the ground including the weight water in the soil before the construction of the structure.

🠞A floating foundation is also known as a balancing raft 

🠞In General when the foundation is not supported by rigid soil mass or rocky strata below the earth’s surface and it rests completely over the soil similar in friction pile then this phenomenon of the foundation is termed as a floating foundation.

🠞Foundation settlement is the major problem faced by geotechnical engineers. If the settlement is extreme, it may lead to the complete collapse of the structure even though the factor of safety against shear failure is high. There are many techniques or ways to reduce the foundation settlement, the floating foundation is one of the best solutions.      

🠞The main application of the floating foundation is to use as a base foundation for sheds, manufacturing workshops, home additional rooms, or garages. It is economical when used in areas wherever the need for a standard foundation is not required.

🠞The floating foundation is called a monolithic foundation as they have no connection with the foundation. The main purpose or aim of the floating foundation is to balance the weight of removed (dragged) soil by a structure of the same weight which causes zero settlement to the structure. For this reason, it is also termed as a balancing raft foundation.

Principle of Floating Foundation: 

The main principle of the floating foundation is to balance the weight of dragged soil by a structure of the same weight which leads to zero settlement to the structure. So, this foundation is also known as balancing the raft foundation.

Principle of floating  foundation

🠞Let’s consider a ground with a water table at the top as shown in the figure. The ground or base is excavated or dragged up to a certain depth which is below the water table. Now in the next step, a building is constructed which is as same as the weight of the removed soil and water.

🠞Even the depth of excavation is below the water table the total vertical pressure in the soil below the foundation is unchanged because of its balancing weight. But here one point should be kept in mind that we cannot build a structure immediately after the excavation.

🠞At the time of construction, the effective vertical pressure below the depth of excavation may slightly increase because of the unbalanced weight. So, this type of floating foundation can also be known as partly compensated foundations instead of fully floating or compensated foundations.

Stability of Floating Foundations:

Floating foundations are best suited for the following types of soils:

🠞The soils which are having good shear strength but having a problem of more settlements and differential settlements due to heavy loads. In such a case, the floating foundation can minimize the settlement values to a greater extent.
🠞The soils which are having lower shear strength and there is no hard layer or strata of soil at a reasonable depth. Here also, the floating foundation is desirable to decrease the shear stresses to a satisfactory level, and hence settlement is prevented.

   Advantages:

🠞The floating foundation can be employed in site areas with lower soil bearing capacity             (SBC) and where investing large money on soil treatment is futile. This system letting us            use over loose soil or in the soil with varying compressibility.
🠞 The floating foundation has the property of spreading the vertical loads or stresses coming       over it to be disturbed over a large area.
🠞 A floating foundation is a good solution when it comes to the extension of houses. It is best suited when we require a building extension without affecting the already existing foundation. 

🠞 This won’t interfere with the building structure already built.

🠞 A floating foundation can be used in high moisture soil.
🠞 The floating foundation does not require the usage of footer trenches. They can be poured       with the help of trenching or digging which is economical.
🠞 It requires very little digging of soil because deep footer trenches are needed.
🠞 It avoids seepage of water and frosting. Due to this purpose, the foundation can be                  insulated for frost or moisture.
🠞 Most suitable for heavily used highways.
🠞 The floating foundation does not cause any disturbance to the earth layer beneath neither         it will interfere with the quality.
🠞 In areas where there are possibilities of shifting in the earth layer, mainly due to high                moisture content, the floating foundation is best suited.

 Disadvantages :

🠞 The footing slab does not make underground land available for underground access for             connection lines facing the utilities.
🠞 It has primitive technology.
🠞The floating foundation has lower design resonance.

Difficulties in Construction of Floating Foundation:

 At the time of the construction of the floating foundation, some problems may arrive which can be prevented by taking proper care. The following are floating slab foundation problems,

1. Excavation: The excavation made for a floating foundation should be done carefully. The walls of the excavated part should be supported using piles or timber piles or soldier piles etc.

2. De-watering: If the depth of the excavation is below the water table then dewatering is essential. It is better to check and establish the water table level prior to the excavation. Dewatering should be done cautiously otherwise it may cause a lowering of water table which disturbs the adjoining structure. Care has to be taken to see that the adjoining structures are not affected by the dewatering process of foundation.

3. Critical Depth: If the soil has low shear strength as specified above, there is a limit to the depth of excavation which is known as critical depth. There are many equations are available to determine the critical depth of excavation among them one is given below known as Terzaghi’s equation.

Where Nc = Skempton’s bearing capacity factor.
Dc = Critical depth
s = Shear strength of soil
B = Width of foundation made (If width is less than the length of foundation “L”)
ᵧ = Unit weight of soil
By using any one the above two equations, the critical depth or maximum depth of excavation can be determined.

4. Bottom Heave: When the soil is excavated up to some depth, the pressure of the soil below this depth is lowered which results in the formation of heave. The formed heave may lead to settlement to the structure or foundation. We cannot prevent the formation of heave but there are some methods to minimize the formation of heave.
Digging narrow trenches and filling them with concrete soon after the excavation minimizes the heave formation. Installing friction piles before the excavation starts or after the completion of half excavation can also prevent the formation of heave under the foundation.By lowering the water table also, it can be minimized. To prevent the heave formation in the sidewalls of the trench, lateral support should be provided. Heaving can be reduced by phasing out excavation in narrow trenches and placing the foundation soon after excavation.